As the printer in an output device of a terminal data communication output machine or computer, widely used is a so-called impact printer of an ink ribbon system for printing on the paper by selectively giving impact to the platen via the ink ribbon and the paper.
Especially, use of a dot matrix printer using wires as the letter printing element has been widespread, since it is small in size and economical.
However, the printer of the ink ribbon system needs to attach an ink ribbon cassette and move it, and accordingly the mechanism is complicated, and especially in the case of printing of various colors the mechanism is made more complicated.
Since the printer have been used in various fields, not only black but also other colors have been demanded. In the ink ribbon system, it is necessary to prepare a number of the printing heads in accordance with the number of colors of the ribbons. Accordingly, this system is mechanically complicated and therefore very costly.
An impact printer that is mechanically simple and prints easily without bringing about the shortcomings of the ink ribbon printer has been developed as seen in FIG. 1 of the attached drawings.
In reference to FIG. 1, porous bodies 5 are impregnated with a plurality of color of ink, and constitute a platen 4 of cylindrical shape which is to be attached opposite to a printing head 1 having wires 2 serving as letter or character printing elements via a printing paper. The platen 4 is rotated and the ink impregnated porous body 5 of a desired color is selected as a printing face. When the printing face is given impact via the printing paper 3 by the wire 2, a printing of the desired color is made on the paper 3.
The platen 4 is formed with the printing face by detachably attaching the ink impregnated porous bodies 5 to a platen base 7 as seen in FIG. 2 thereby making it easy to exchange the bodies 5 in case the ink is used up or any of the bodies are damaged. Further, it is possible to exchange one set of the platen having the ink impregnated elements of other different colors.
In the embodiment shown in FIG. 2, magnets 9 are employed as means for attaching the ink impregnated porous bodies 5 to the platen base 7. The magnetic piece 9 is positioned into a groove 8a defined in an attaching face 8 of the platen base 7, and on the other hand the ink impregnated porous body 5 is secured at its bottom with an iron plate 6 and is attached to the platen base 7 due to magnetic attraction.
The porous body is in general composed of a sintered material of polyamide resin of around 0.70 to 0.85 in density which is impregnated with ink. In printing, depending on the density, either durability or formability are provided. When a large quantity of ink is contained therein, the body reveals a color darker than the one actually used.
Therefore, it is not easy to distinguish a desired color for the printing paper from the outer appearance of the ink impregnated porous body.
Further, in such a case that the porous body is impregnated with light colors such as yellow, red or light blue, its surface is often stained with darker ink such as black, purple or dark blue. This stain does not give bad influences to the printing at all, but the porous color body is made unsightly.
For removing such bad conditions, it is assumed that the porous body itself is made black, which makes the color of the impregnated ink absolutly unclear.